Phase-cut dimmers are widely used for dimming incandescent types of lighting. When applied to solid state lighting, however, the results of using phase-cut dimmers are often unsatisfactory. When used with light emitting diode (LED) lighting the dimmer can produce a perceptible and annoying flicker or shimmering of the light level. Such variations in light intensity (collectively referred to as flicker) are usually more noticeable when the light is dimmed to lower levels and are generally less noticeable or even imperceptible at high or undimmed light levels.
A solid state LED lighting device typically includes an LED driver that converts AC power to regulated current to drive the LED loads. In drivers whose switching is based on the AC line voltage, the average LED current may not be consistent from cycle to cycle due to dimmer phase jitter. The variation of load current causes visible shimmer, particularly when dimming to low light. The load current variation resulting from phase jitter can come from many sources, such as AC line transient voltages or currents, phase-cut dimmer instability, or control variation. The amount of jitter is usually small enough as a percentage of the total power to not attract attention with little to no dimming at brighter light levels. But with dimming to low light levels, the percentage of current change due to the variation is higher and the resulting flicker or shimmer is more perceptible. In addition, the flicker can be random in amount or intensity or occur at certain times or randomly due to the unpredictable nature of the source of the variation. The variation and resultant shimmer can be more perceptible and irritating at lower frequencies such as about 4 Hz to 8 Hz. Another example that occurs with phase cut dimming is that at low light dimming levels the LED current will be small and tend to follow the AC line levels at 60 or 120 Hz so that the low frequency jitter on the AC line will cause a visible flicker or shimmer.
A common way to reduce flicker and shimmer is to add a ripple reduction circuit, e.g., using either discrete components or an integrated circuit. The purpose of such a discrete or integrated circuit, however, is to reduce ripple (50-120 Hz) at the input power supply, not shimmer and flicker (4-8 Hz) at the load. Such a circuit filters out not only the low light shimmer and flicker but also the normal output current ripple at higher loads. As described above, however, the flicker reduction function is not needed with little to no dimming at brighter light levels at higher loads, because when the output current is large, the impact of the low frequency variation is much less than when the output current is low. This application of ripple reduction circuits to reduce flicker and shimmer at low load levels as well as high load levels is wasteful of energy compared to using a circuit that more specifically suppresses shimmer and flicker at low load levels.
Accordingly, there is a need in the art for high efficiency flicker attenuators for solid state LED lighting.